1. Field of the Invention
This invention relates to seismic wave generating devices utilized in seismic exploration, and more particularly, to seismic wave generators operating upon the ground impacting principle, and utilizing a ram which is driven into forceable contact with the earth, or with an intervening impact distribution structure, such as a metal plate.
2. Brief Description of the Prior Art
Many types of seismic wave generating devices previously utilized have included a seismic vibrator or sound generating device mounted on a vehicle, and including an elongated ram which is driven forceably against the earth to generate sound waves which are propagated into the ground for the purpose of developing a seismic profile based upon the detection of reflected waves which are returned to the surface from various subterranean formations. Various methods and apparatus have been employed for propelling the ram forceably against the earth. In some instances, a pneumatic or hydraulic drive is employed to force the ram from an elevated position to a lowered position in contact with the earth. In other types of apparatus, large compression springs are loaded in compression upon elevation of the ram by hydraulic or pneumatic means and, upon release of the ram, function to propel the ram forceably against the earth. Examples of such compression spring actuated ram-type seismic wave generators are those depicted and described in Williams, U.S. Pat. Nos. 3,193,046; 3,209,854 and 3,270,832. Compression spring actuated seismic wave generators of this type are characterized by a propensity for the ram to undergo some rebound after striking the earth, followed by a second less forceable impact. This develops undesirable secondary waves which interfere with the obtainment of the most reliable and accurate read-out at the surface to provide meaningful information on the character of the various subterranean formations penetrated by the sound waves.
Many of the types of the earth-impacting seismic wave generating apparatus which have previously been used provide means for better distributing and directing the sound waves into the earth. In some instances, structures for accomplishing such distribution and direction have included a plate secured to the lower end of the ram, and having a substantial surface area, so that the area of the earth impacted upon descent of the ram is substantially larger than the diameter of the ram per se. In other instances, it has been proposed to direct the end of the ram against a metallic plate rested on the surface of the earth in alignment with the direction of the ram stroke. A disclosure of this type of structure appears in Kenney U.S. Pat. No. Re. 26,825. The three Williams patents previously referred to also include, generally, a steel anvil which is interpositioned between the compression spring reciprocated ram and the earth.
Some advantage has been thought to accrue from the use of a series of superimposed or nesting steel plates which are interconnected by chains, and which are permitted to fall by gravity to the earth so that, upon impact, the plates, in sequential fashion impact upon each other and come to assume the nested relationship. This multi-section or laminated weight plate structure is indicated by the patentee to provide an improved and optimized seismic wave of effective pulse shape and frequency.